Many techniques exist for measuring the concentration of a constituent gas or gases in a gas sample. Non-dispersive infrared (NDIR) techniques infer the concentration of a constituent gas by measuring a gas sample's ability to absorb electromagnetic radiation. By examining absorption in a wavelength band where absorption is dominated by one constituent gas, relatively simple NDIR sensors can accurately measure concentration of that gas as a function of radiation absorption. For instance, carbon dioxide (CO2) detectors often measure absorption at a 4.2-micron wavelength, where CO2 strongly absorbs. For other gases, other visible or infrared bands can be selected.
With appropriate focusing optics, free-air absorption of atmospheric gases is possible. Nevertheless, most environmental NDIR sensors utilize enclosed gas sample chambers to, e.g., enhance the signal received at the detector, prevent contamination of the optical components, ruggedize the device, make the device smaller and more portable, etc.
FIG. 1 illustrates a typical prior art gas sample chamber 10, similar to that disclosed in U.S. Pat. No. 5,163,332, issued Nov. 17, 1992, to Wong. Chamber 10 comprises a specularly reflective tube 12 with a source 16 (e.g., an incandescent bulb) at one end and a radiation detector 18 at the opposite end. An array of circular apertures 20, drilled along tube 12, allows gas to enter and exit chamber 10. Each aperture is covered by a semipermeable membrane 22 that filters particles down to at least 0.1-micron particle sizes, to prevent those particles from entering the chamber and diminishing the critical reflectivity of inner chamber surface 14.
As is typical in the prior art, the mirrored chamber walls of tube 12 act as a light pipe, guiding radiation, emitted by source 16, down to detector 18. Light ray 24 represents a boresighted light ray that passes directly from source 16 to detector. Light rays 26, 28, and 30 represent light emitted at increasing angles of deflection, as measured from the boresight angle. Ray 26 reflects once off of the tube wall—with a tube aspect ratio of 12:1, ray 26 has a deflection angle of about 4.8 degrees. Ray 28 reflects twice off of the tube wall—with the same tube aspect ratio, ray 28 has a deflection angle of about 9.5 degrees. Ray 30 reflects three times off of the tube wall, and has a deflection angle of about 14 degrees. Higher-order reflections are possible, but tend to be less effective, due primarily to the difficulty of avoiding intersection with one or more of the apertures 20 (which do not propagate light forward) as well as diffusive and reflectivity losses that multiply with repeated reflection. Accordingly, most of the light that reaches detector 18 emanates from source 16 within about a plus-or-minus 14-degree cone surrounding the boresight angle.
A competing sample chamber 40, designed by the inventor of the present invention, is shown in FIG. 2. Instead of an elongated tube, chamber 40 has a relatively short aspect-ratio (e.g., 4:1), and is formed in a relatively thick-walled stock material 42. Four gas ports 44, 46, 48, and 50 are bored through the chamber walls, two near the source and two near the detector. Port 50 is used for injection of calibration gas, and is nominally blocked by cap 52. Ports 44, 46, and 48 allow different two- and three-port forced-flow and gas diffusion configurations—a three-port gas diffusion configuration is shown. Each open port is covered with some sort of porous media 54 to prevent insects, spiders, gross dust, and large solid objects from entering chamber 40.
Sample chamber 40 accepts a source assembly, comprising incandescent bulb 56, elliptical reflector 58, and mounting plate 60, into a machined hole in one end. Chamber 40 accepts a detector assembly, comprising an aperture stop 62, a detector 64, and a mounting plate 66, in a second machined hole in its opposite end.
One significant difference between light pipe chamber 10 and chamber 40 is the surface finish of the inner chamber. Instead of a mirror surface, chamber 40 incorporates an inner surface 68 that is relatively rough compared to radiation at wavelengths of interest. During fabrication, the surface roughness and stability are enhanced by a surface etch and a yellow chromate step.